In general, an electroluminescent element includes an inorganic electroluminescent element using an inorganic compound in a light emitting element and an organic electroluminescent element using an organic compound in a light emitting element. In recent years, active studies on practical realization of the organic electroluminescent element have been made because the element can achieve light emission at a low voltage and at a high luminance.
A basic structure of the organic electroluminescent element is obtained by forming a hole injection layer and an organic thin-film layer such as a light emission layer on a glass plate deposited with a thin film of an anode material such as indium-tin oxide (ITO) and further forming a thin film of a cathode material thereon, and there is known an element obtained by appropriately providing a hole transport layer or an electron transport layer on the basic structure. A construction of layers in the organic electroluminescent element is, for example, anode/hole injection layer/light emission layer/electron transport layer/cathode or anode/hole injection layer/hole transport layer/light emission layer/electron transport layer/cathode.
In recent years, it has been found that when charge transport layers such as the hole injection layer and the hole transport layer are integrated between the light emission layer and the anode, the layer improves an ability to inject holes into the light emission layer and serves as a buffer layer that optimizes a charge balance to significantly improve light emission efficiency and life of the element.
Hole transporting materials used in the hole transport layer of the organic electroluminescent element are broadly classified into a low-molecular-weight hole transporting material and a high-molecular-weight hole transporting material.
As a method of forming the low-molecular-weight hole transporting material into a film serving as the hole transport layer, a vacuum deposition method is mainly used and has the following characteristics. According to the method, it is easy to produce a multilayer using various materials having different functions, which allows a high-performance organic electroluminescent element to be formed. However, there is a problem in that it is difficult to control a thickness uniformly and to apply different materials depending on parts for achieving a large-screen and high-definition panel, and a large-size vacuum apparatus is required, resulting in an increase in production cost.
In addition, as the method of forming the low-molecular-weight hole transporting material into a film serving as the hole transport layer, a film formation method involving application of a solution containing the low-molecular-weight hole transporting material has been studied toward practical use. However, it is necessary to improve this technique for practical use because segregation and phase separation due to crystallization of the low-molecular-weight compound are observed.
On the other hand, as a method of forming the high-molecular-weight hole transporting material into a film, a solution application method such as a spin coating method, a printing method, or an ink-jet method is used because most of the material cannot be deposited by the vacuum deposition method. The method facilitates an increase in screen size and is excellent in mass productivity. In contrast, the method involves the following problems. Layers are liable to mix with each other and hence function separation by lamination cannot be performed. In addition, properties different from those required in a dry process such as solubility in a solvent are required, and hence a charge injecting material and charge transporting material that can be used in a wet process are limited.
As attempts to express such required properties, for example, Patent Literature 1 reports an acrylic compound or a cured product thereof, and Patent Literature 2 reports a cured product using an NPD having a vinyl group. Although function separation by lamination is achieved in an organic electroluminescent element using any such compound, its electron resistance and charge transporting performance are not sufficient, and hence the element has not obtained sufficient properties.
In addition, as a technique of enhancing the light emission efficiency of the organic electroluminescent element, a polymer material having a main chain of a n-conjugated polymer including an indolocarbazole unit excellent in electron resistance and charge transporting performance integrated thereinto, and a light emitting element including the polymer material have been disclosed. That is, Patent Literature 3 discloses a conjugated polymer obtained by bonding an indolocarbazole at 6- and 12-positions, and Patent Literature 4 discloses a conjugated polymer having an N-position substituted indolocarbazole as a main skeleton. Those polymers each improve the electron resistance and the charge transporting performance. However, the π-conjugated polymer containing an indolocarbazole skeleton in its main chain involves the following problem. The polymer has low solubility in an organic solvent and hence it is difficult to form the polymer into a film. Even when the polymer can be formed into a film, the thin film itself does not have any solvent resistance as in any other polymer that can be applied, and hence any other material such as a light emission layer material cannot be formed into a film on the film by an application method after the film formation.